One of the important features of integrated circuits deigned for portable applications is their ability to efficiently utilize the limited capacity of the battery power source. Typical applications include cellular telephones and personal digital assistants (PDAs), which might have a Lithium ion battery or two AAA alkaline batteries as the power source. Users have come to expect as much as three to four weeks of standby operation using these devices. Standby operation refers to the situation where the cellular phone, handheld device, etc. is powered on but not being actively used (e.g., actively involved in a call). Generally, it is estimated that that the integrated circuits providing the functionality of the device is only performing useful work approximately 2% of the time while the device is in standby mode.
Removing the power supply from selected circuits of a device during standby is a technique employed by designers for battery powered applications. The technique is generally applied only to circuit blocks outside of the central processing unit (CPU). A primary reason for not applying this technique to CPUs, has been the difficulty in being able to retain the current processor state information necessary to continue execution after coming out of the standby mode. One solution for this limitation involves saving the current processor state information to external storage mechanisms (e.g., such as flash memory, a hard disk drive, etc.). In such a case there is the overhead required in transferring the state to and from the external storage mechanism. Even if the battery powered device had a hard disk drive, and many don't, the time consuming state transfer may not meet the real time response requirements of the application when the device needs to wake up to respond to a new event.
Other solutions involve the use of specialized DRAM components that are configured to maintain their own refresh states. Such components incorporate mechanisms for refreshing volatile DRAM memory cells without interaction with external memory controllers, as would be the case where a memory controller shuts down during sleep mode. As with saving CPU state, another solution would be to transfer the contents of volatile DRAM to non-volatile memory (e.g., Flash, disk storage, etc.) prior to entering sleep mode.
Power consumption during active mode is another important feature, particularly for battery-powered band held electronic devices. In addition to the problems involved in placing a system into sleep mode and reliably waking the system upon exit from sleep mode, there have been a variety of different efforts to reduce power consumption during the active modes of device operation. Such efforts include, for example, utilizing specialized low-power processors that are specifically configured for battery-powered handheld devices. Unfortunately, “low-power” processors are often “low performance” processors, which force compromises on the usability and the responsiveness of the user experience.
Thus, what is needed is a solution for powering down an electronic device for reduced standby power consumption while retaining the ability to quickly resume full power operation. What is further needed is a solution for reducing the power consumption of an electronic device while the device is actively executing a user application.